This invention relates generally to data recovery systems and more particularly to systems utilizing partial response maximum-likelihood (PRML) detection.
Data recovery systems are used in a variety of applications. In one application, such system is used to recover (i.e., read) data stored on a magnetic disk of the type used as hard drives in computer data storage systems. In one type of such data storage system, a predetermined preamble pattern is written onto the magnetic disk before the start of any block of data to be stored. The preamble pattern is used during the read operation to acquire initial gain, frequency and phase lock to the data read from the disk. Thus, when data is read from the disk, the preamble is used by the read channel during a preamble recovery mode to obtain clock (i.e., sampling or timing) pulses for a subsequent data recovery mode.
The data read from the magnetic disk is passed through an analog equalization filter prior to passing to succeeding stages of the data recovery system. The analog equalization filter is configured to shape the sequence of preamble pulses and data pulses into pulses having a predetermined waveform characteristic. The analog equalization filter has a non-linear change in phase as a function of frequency characteristic. The analog equalization filter produces pulses that are fed to an analog to digital converter. The analog-to-digital converter converts samples of the shaped pulses into corresponding digital words in response to the clock pulses (i.e., sampling pulses) fed to a clock input of the converter. One such arrangement is shown in U.S. Pat. No. 5,220,466.
Direct current (DC) offsets may be introduced into the data recovery process by analog signal processing circuitry, such as amplifiers and filters. The DC offsets of theses devices may vary as a function of temperature and signal gain. The DC offset associated with these non-ideal devices results in a signal level shift which, if uncorrected can degrade performance of the data recovery system. One technique suggested to compensate for this DC offset is discussed in U.S. Pat. No. 5,459,679.